Cyclic adsorption processes, such as pressure swing adsorption (PSA) and temperature swing adsorption (TSA) are efficient methods for separating the components of a gas mixture using a particulate adsorbent material which adsorbs one or more components of the gas mixture at elevated pressures and low temperatures more strongly than it adsorbs one or more other components of the gas mixture. The process is generally carried out in one or more elongate adsorbent-packed vessels having a feed gas inlet and a nonadsorbed gas outlet by a multi-step cycle which includes an adsorption step and a bed regeneration step. During the adsorption step the gas mixture is passed cocurrently through the bed (in the direction from the gas inlet toward the gas outlet). As the gas passes through the bed of adsorbent the more strongly adsorbed component is adsorbed while the less strongly adsorbed component passes through the bed. As the adsorption step proceeds, the strongly adsorbed component forms a front which gradually advances toward the outlet end of the bed. The adsorption step is terminated before the adsorbed gas front breaks through the bed of adsorbent, and the adsorbent then undergoes regeneration, during which the vessel is depressurized by the countercurrent discharge of the adsorbed gas from the vessel (PSA processes), or the adsorbent is heated sufficiently to cause desorption of the adsorbed components from the adsorbent (TSA). The process is usually practiced in two or more adsorption vessels operated out of phase such that the adsorbent in one vessel is in adsorption service while the adsorbent in another undergoes regeneration.
PSA and TSA processes are ideally suited to the separation of components of gas mixtures which contain relatively constant concentrations of the component being adsorbed. This is particularly important in cyclic adsorption processes carried out in multi-vessel systems, since constant concentration of strongly adsorbed component in the system feed gas is highly desirable or necessary for the attainment of steady state conditions, which makes it easier to produce a gas product of more uniform purity. Also, when the concentration of the strongly adsorbed gas component varies over a wide range it is difficult to avoid the occasional occurrence of breakthrough of strongly adsorbed component into the nonadsorbed product gas stream.
In certain chemical processes waste gas streams are generated and discharged from the reaction zone on a substantially continuous basis but certain components of the waste gas are only produced sporadically during the course of the processes. For example, some semiconductor fabrication processes use low molecular weight fluorinated compounds in combination with oxygen for etching of silicon chips and for cleaning residue from the chemical vapor deposition chambers. These processes are typically intermittently conducted under vacuum. The exhaust gas from the chamber contains, in addition to the fluorinated compounds, unreacted deposition compounds and a variety of reaction products, such as hydrogen fluoride, silanes, halides, etc. Since these compounds cannot be safely released to the atmosphere the exhaust gas is generally treated to destroy the potentially harmful compounds or convert them to compounds that can be released to the atmosphere.
According to one procedure, the deposition chamber exhaust gas is combined with an inert carrier gas, e.g. nitrogen, upstream of the vacuum pump used to evacuate the chamber, and mixture is introduced into a gas reactor column (GRC) such as the column manufactured by Edwards High Vacuum International Division of The BOC Group, plc under the trade designation EDWARDS GRC, wherein components of the stream are reacted at high temperatures and converted into disposable solid substances. Fluorinated compounds are highly nonreactive, however, and pass through the reactor unaffected, together with the inert carrier gas. The fluorinated compounds are recovered from the GRC waste gas by PSA or TSA using adsorbents which selectively adsorb fluorinated compounds. This process is described in U.S. Pat. No. 5,417,742, the text of which is incorporated herein by reference.
A problem encountered in the above described process results from the intermittent practice of the silicon chip etching or deposition steps and vapor deposition chamber cleaning steps. These are conducted on an alternating basis; accordingly, although nitrogen is continuously introduced into the waste gas stream from the chamber, the concentration of fluorinated compound in the waste gas stream is irregular because of the intermittent discharge of the perfluorocarbons from the vapor deposition chamber. These conditions make it very difficult to efficiently remove the fluorinated compounds from the waste gas stream by PSA or TSA.
Because of the low cost and high efficiency of PSA and TSA processes for recovering strongly adsorbed compounds from gas streams, improvements which permit these techniques to be used under diverse conditions are continuously sought. The present invention provides a procedure which makes it possible to efficiently and effectively separate, by a cyclic adsorption process, continuous flow gas mixtures which contain varying concentrations of strongly adsorbed components.